In a controlled environment such as a laboratory setting, it is a common practice to utilize a gas-filled, three-terminal type capacitor for measuring high voltages. Such a capacitor exhibits a relatively low loss factor and is not influenced by external fields, both characteristics being useful to effect precise measurements in laboratory test set-ups.
Representative of these conventional capacitor types is the arrangement disclosed in U.S. Pat. No. 3,829,742 issued to Muller on Aug. 13, 1974. In this arrangement, cylindrically-shaped high and low voltage electrodes are disposed in coaxial relationship within a sealed chamber or housing. The high voltage electrode is suspended from a combined high voltage bushing-lead structure which forms the top of the chamber. The low voltage electrode, which encompasses the high voltage electrode, rests on a layer of insulation material seated on a floor plate of the chamber. A low voltage lead is introduced into the chamber through an insulated bushing formed integrally with the wall of the chamber. The chamber is grounded to mitigate the effects of external fields. The interior of the chamber is filled with a gas under pressure.
This type of gas-filled capacitor exhibits a phenomenon, called "statistical breakdown" in the high-voltage art, wherein an internal flashover or arcing between the electrodes may occur at a random time. The flashover becomes, in effect, a short between the electrodes, leading to a very high surge of current from the high voltage line attached to the capacitor through the electrodes to ground. In turn, this current flow generates heat internal to the capacitor, thereby causing further pressurization of the gas. If left unchecked as is the case with a field installation, specifically a connection to a power line, an explosion may occur. In the laboratory, it is possible to substantially reduce such current surges by utilizing external current-limiting circuitry. Because of controlled conditions and stabilizing efforts possible in the laboratory, dangerous situations such as explosions are precluded.
However, while useful in the laboratory, the gas capacitor may be susceptible to the deleterious effects of a harsh field environment, particularly in revenue metering applications wherein the capacitor is permanently installed on a customer premises. For instance, because of ambient temperature variations, the capacitor components are subjected to degrading influences over time, thereby increasing the likelihood of a flashover. It is impractical to provide in the field setting the sophisticated current limiting circuitry deployed in the laboratory because of cost and complexity. Moreover, pressurized gas components attached to a power line are bulky and complicate the accessibility and work activity of craft personnel.
Also, in the laboratory, an independent voltage source is readily available to energize ancillary circuitry which may then be used in support of measurement applications utilizing the three-terminal capacitor. In the field, however, it would often be preferable to derive a voltage signal for auxiliary circuits directly from the high voltage source being measured.
In revenue metering applications for power utility companies, it is necessary to derive both a current and voltage indicative of power usage in order to measure the usage. Current detection has been effected with a current transformer which is not formed integrally with the three-terminal capacitor. This is costly and proliferates the number of distinct components placed along the transmission line.